1. Field of the Invention
The present invention relates to nondestructive eddy current testing devices, and more particularly to such devices capable of measuring multiple variable characteristics of a sample.
2. Description of the Prior Art
Conventional nondestructive eddy current testing devices typically utilize a test coil which is placed in proximity to a sample excited by a signal of a single frequency. Single frequency excitation results in an output from the test coil which may be utilized to determine unambiguously the value of two characteristics (i.e. metal plate thickness and electrical conductivity), but not more than two variable characteristics or parameters of the sample. Furthermore, even in applications where additional characteristics of the sample are not of direct interest, their affects on the test coil output may mask characteristics of interest and thereby reduce the effectiveness of the eddy current test. Although in some cases the information obtained from such devices is sufficient, in general, much of the test information concerning the sample remains unrevealed.
To increase the information obtainable from eddy current testing, devices have been developed employing a test coil excited by a multifrequency signal, as exemplified by U.S. Pat. No. 3,229,198 to Libby, or by various single frequency signals applied sequentially, as exemplified by U.S. Pat. No. 3,391,336 to Henschel. These devices typically employ analog computation circuitry and require a measurement of the amplitude and phase of each frequency component of the test coil output in order to resolve the output into a representation of sine and cosine signals.
In the device of Libby, a multidimensional excitation current, described in one embodiment as two superimposed sinusoids, is applied to a test coil. A pair of narrow bandpass filters each receive a portion of the test coil output signal and feed their respective filtered outputs to an amplitude-phase detector circuit. These latter circuits each produce a signal in phase and a signal in quadrature with a fixed reference signal. These four signals, or descriptors, represent the Fourier expansion of the test coil output signal. Hence, the device of Libby expands the test coil output signal on a set of basis functions. Linear potentiometer summing circuits in turn receive these descriptors and are adjusted in a complex one-step manner to eliminate the effects of certain parameters and provide outputs indicative of desired parameters.
The device of the Libby patent, however, is not easily extended to applications in which more than four such descriptors are needed in order to determine the desired information. To obtain a greater number of usable descriptors, the test coil of Libby must be excited by a signal comprised of more than two frequencies. Consequently, more than two narrow bandpass filters will be required for separating the test coil output signal into its respective frequency components, and a corresponding increase in the number of amplitude-phase detection circuits will be required. In addition to the increased complexity, increased cross talk, or coupling, occurs and interferes with the operation of the device.
The problems associated with adjusting the potentiometer summing circuits of the Libby device are somewhat reduced by replacing these circuits with an array of transformation rotators as disclosed in U.S. Pat. No. 3,706,029 to Wandling, et al. However, the arrangement of rotators of Wandling requires a still somewhat complex iterative adjustment of the rotators.
One common drawback of all known eddy current testing devices is their lack of employment of binary functions. This deficiency limits the extent that such devices may utilize digital circuitry and in turn adds to their complexity and cost. Furthermore, for the sake of speed and simplicity in the realm of computers, it is desirable that such devices maximize the extent that they work with binary functions.
It has been discovered that a set of binary functions called Walsh functions may be used as a basis for expansion of an eddy current signal. As a result of this discovery, a relatively simple eddy current testing device has been designed which may be used in a wide variety of eddy current testing applications, including those in which a multiple frequency signal is used for excitation of a test coil.